In recent years, voice communications using various kinds of networks become very popular, but telephonic voice communications generally use the limited frequency band, i.e., the so-called telephone band that has a frequency range from 300 Hz to 3.4 kHz according to the practice of the time when the conventional general public networks were being used. However, a human voice also includes a frequency component below 300 Hz and another frequency component above 3.4 kHz, which are important components relating to individuality of utterance, and a lack of these components leads to not only the loss of individuality but also the degraded voice quality. Therefore, it is desirable that the telephonic voice communication be carried out using a voice including these components.
However, there is a problem that a telephone exchange in the public switched telephone network cannot transmit the voice components outside the telephone band. Furthermore, there is another problem that a sending-side terminal in a network other than the public switched telephone network is designed according to the conventional practice not so as to be able to transmit the voice components outside the telephone band. There are proposals for some technologies relating to such problems, which are disclosed in, for example, Patent Document 1.
The technologies disclosed in Patent Document 1 will be described with reference to FIG. 2. Referring to FIG. 2, first, a narrow-band signal DC, a frequency of which is limited to a range from 300 Hz to 3.4 kHz, is input to the band extender 10.
In the band extender 10, the narrow-band signal DC is input to the sampling frequency converter 11 to be converted to a converted original signal S, a sampling frequency of which has been changed. The converted original signal S is then supplied to the high range signal generator 13, the voiceless portion signal generator 14, and the low range signal generator 12 respectively.
The low range signal generator 12 generates an extended signal (hereinafter referred to as a “synthesized low range signal”) LS including components extended to a lower frequency side (below 300 Hz) from the converted original signal S. The high range signal generator 13 generates another extended signal (hereinafter referred to as a “synthesized high range signal”) HS including components extended to a higher frequency side (from 3.4 to 7 kHz) from the converted original signal S. The voiceless portion signal generator 14 generates another extended signal (hereinafter referred to as a “synthesized voiceless signal”) US including an extended voiceless portion (a high range voiceless portion in the case of Patent Document 1) from the converted original signal S. Furthermore, the adder 15 adds the synthesized low range signal LS, the synthesized high range signal HS, and the synthesized voiceless signal US to the converted original signal S, thereby generating a band extended signal V.
Since the band extended signal V is generated by supplying both the low range signal and the high range signal generated from the band limited narrow-band signal DC in addition to the transmitted signal, a user can hear a voice of the band extended signal V that makes him/her feel as if he/she heard a voice of the wide-band signal including these components.
The generation of the synthesized low range signal LS is carried out by generating a fundamental period waveform having a period based on a fundamental frequency of the extended low range from an autocorrelation function. Further, the generation of the synthesized high range signal HS is carried out by generating an arbitrary sound source waveform on the basis of a result of the estimation of a fundamental frequency and an amplitude and extracting only the high range components therefrom. Furthermore, in Patent Document 1, the fundamental period waveform and the amplitude are obtained from the low range signal generator 12, but they may be estimated in the high range signal generator 13 separately. Moreover, the generation of the synthesized voiceless portion signal US is carried out by extracting a high frequency component of an aliasing (folding noise) in a half-wave rectification and extracting a high range component not having a harmonic structure.
Patent Document 1 is Japanese Patent Application Kokai (Laid Open) Publication No. 9-258787.